Vibration Frequencies and Physiological Responses – NIOSH

“Speaking on April 19 at the Experimental Biology 2009 meeting in New Orleans, Dr. Kristine Krajnak, a team leader in the Engineering and Control Technologies Branch of the Health Effects Laboratory Division of NIOSH in Morgantown, West Virginia, describes results from the first study to directly link the different physical responses of tissue that occur with exposure to different vibration frequencies with biological mechanisms underlying the development of vascular dysfunction. Her presentation is part of the scientific program of The American Physiological Society.

The study, along with results of other studies conducted by NIOSH, supports the importance of reducing job-related exposure to vibration. Ongoing research is evaluating the effectiveness of anti-vibration devices, such as anti-vibration gloves and tools.

Higher frequency vibrations produced by an electric sander (greater than 100 Hz) are smoother than the slower vibrations of an electric hand drill (approximately 63 Hz) and therefore are less likely to cause users discomfort.

Don’t let that fool you into not using protective devices that can reduce your exposure to vibration, she says. The new research study conducted at the National Institute for Occupational Safety and Health (NIOSH) suggests that exposure to high and low frequencies cause different physiological responses, but both may affect the risk of developing vibration-induced peripheral vascular dysfunction.

Of the 1.1 to 1.5 million U.S. workers exposed to hand transmitted vibration on a fairly regular basis, approximately half eventually develop some disorder such as Vibration White Finger, in which a single finger or sometimes the entire hand turns white and numb when exposed to the cold, due to restricted blood flow.

Workers also may experience reductions in tactile sensitivity, grip strength, and/or manual dexterity. Earlier studies have shown that risk goes up with frequency and duration of exposure, although NIOSH studies are underway to determine why certain people appear more susceptible to shorter exposure durations.

Dr. Krajnak’s team looked at two aspects of vibration injury about which very little is known: the mechanisms of injury and the differences in response to frequency of vibration. The researchers used rats, since the tissues, nerves and arteries of rat-tails are similar to those in human fingers and the tails are known to respond to vibration in a way similar to that seen in fingers.

For four hours a day (the longest time a human can be exposed to workplace vibration according to U.S. and international standards) for 10 days, 15 rats (five in each group) were placed in a container where their tails were vibrated at either 63, 125 or 250 Hz. One control group of five rats accompanied them to the experimental area, to make sure any results seen were not related to noise or change of locale. A second control group stayed in their home cages, uninvolved in the activity.

After the last exposure, the scientists examined the tail arteries for changes. Neither control group had changes, suggesting the changes seen were directly related to the effects of vibration. The rats that experienced high frequency (125 and 250 Hz) vibration had higher levels of measures of oxidative stress, while rats that experienced the lower frequency (65 Hz) vibration showed higher levels of pro-inflammatory factors.

The changes seen following higher frequency vibration are associated with more immediate changes in the peripheral vascular system, such as those seen in workers with vibration injury, says Dr. Krajnak, but the changes following lower frequency vibration also can lead to vascular problems.”